Conventionally, there is a sheet processing apparatus for executing a post-processing on a sheet conveyed from an image forming apparatus. The sheet processing apparatus includes a processing tray, a longitudinal alignment mechanism and a horizontal alignment mechanism. The processing tray is used in the post-processing of the sheet. The longitudinal alignment mechanism has a rotatable paddle. The longitudinal alignment mechanism can align the sheets placed on the processing tray in a sheet conveyance direction. The horizontal alignment mechanism has a horizontal alignment plate movable in a sheet width direction orthogonal to the sheet conveyance direction. The horizontal alignment mechanism can align the sheets placed on the processing tray in the sheet width direction. However, if the sheet is moved by the horizontal alignment mechanism in the sheet width direction, or depending on a sheet size, a position or number of paddles contacting with the sheet changes. If the position or the number of paddles contacting with the sheet changes, a force (hereinafter also referred to as a “longitudinal alignment force”) for striking the sheet of the paddle changes. If the longitudinal alignment force changes, there is a possibility that the sheet skews and misalignment occurs.